To ensure a high-speed circulation of a laser gas in a gas circulation-type laser device, a Roots blower having a gearbox filled with oil is employed, and therefore, oil vapor from the gearbox tends to adhere to the optical components of a laser oscillator device. Thus the life of these components is shortened.
Therefore, during the operation of the gas laser device, gas is continuously exhausted from the gearbox filled with oil, to create a vacuum therein and thus prevent the oil from entering the discharge tube and the like in which the laser gas is circulated. On the other hand, when the operation of the gas laser device is stopped, the gearbox is filled with the laser gas to a pressure equivalent to the atmospheric pressure, to prevent oil from entering the discharge tube and the like while the device is inoperative.
The process of filling the laser gas to the atmospheric pressure level is executed manually by an operator, and therefore, problems arise in that the operator may omit the filling process or may turn off the power supply before the atmospheric pressure level is reached, with the result that the laser gas is not filled to a predetermined pressure level.